1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a recording material.
Especially, the invention relates to an image forming apparatus having a fixing member which heat-fixes a toner image on a recording sheet to a recording sheet by a fixing nip, and a fixing apparatus using a pressure member which comes into contact with the fixing member and forms the fixing nip.
2. Description of the Related Art
In recent years, on-demand print market which prints necessary number of commercial printed materials such as catalogs, posters, brochures and the like, or continuous prints while changing a portion of contents of print such as various debit notes and direct mails is increasing. In the on-demand print market also, an electrophotographic system image forming apparatus is in the limelight in place of offset print which needs plate-making.
Hence, in the electrophotographic system image forming apparatus, such an on-demand print, i.e., a fixing apparatus having a fixing nip which is long in a conveying direction of recording sheets is employed to meet high speed requirement.
However, if the fixing nip is made long, since an amount of heat that a pressure member receives from a fixing member is high, the temperature of the pressure member is prone to rise, and the temperature of the pressure member excessively rises and image failure is generated in some cases. As one example of such image failure which is generated due to excessive temperature rise of the pressure member, there is a phenomenon called “blister”. This phenomenon is generated when a coated paper sheet is used as the recording sheet. If the coated paper sheet is excessively heated, moisture in a base layer in the coated paper sheet is evaporated, and this water vapor bursts through a weak portion of the coat layer and is exposed.
Especially, as shown in FIG. 20, if a distance between paper sheets (a distance between a leading recording sheet and a subsequent recording sheet in a conveying direction of the recording sheet, or time elapsed until the subsequent recording sheet enters the fixing nip after the leading recording sheet passes through the fixing nip) becomes long during continuous printing operation, time during which no recording sheet exists in the fixing nip becomes long. As a result, the temperature of the pressure member rises excessively.
Hence, Japanese Patent Application Laid-open No. 2005-316397 prevents the excessive temperature rise by temporarily separating the pressure member from a fixing roller member when a distance between sheets becomes long during the continuous printing operation.
According to the apparatus disclosed in Japanese Patent Application Laid-open No. 2005-316397, however, since the problem is solved by the temporary separation of pressurizing operation, relatively long time is required for separating the pressure member and subsequent re-abutting operation against the fixing member.
That is, in order to suppress the temperature rise of the pressure member when time corresponding to a distance between sheets during the continuous printing operation is relatively short, the problem can not be solved by the temporary separation motion of the pressure member.
This is because that if the problem is to be solved by the temporary separation of the pressure member, time required for separating the pressure member and subsequent re-abutting operation against the fixing member becomes longer than time corresponding to a distance between sheets, and this deteriorates throughput of the image forming operation.